Practitioners in technological fields involving metal-to-metal interface employ terms of art relevant to the understanding of the present invention and the prior art over which it constitutes an improvement. For example, an explosive weld connotes the metallurgical bond created at the point of impact when one metal is driven against another by the force of an explosion. An explosive weld is distinguished, for example, from a friction weld, i.e., the metallurgical bond created between two metals when they are rubbed together under high pressure conditions. A dissimilar metal sheet is a sheet of metal consisting of two or more layers of dissimilar metal which have been joined together by, for example, explosive or friction welding. A transition bushing is a metal-to-metal interface bushing fabricated from a dissimilar metal sheet.
Similar metals may be interfaced with each other by standard procedures, such as laser welding, soldering or the like. Dissimilar metals, e.g., metals characterized by differing thermal expansion properties, melting point, weld incompatibility or the like, do not reliably interface using such standard procedures. For example, iron cannot be physically laser welded to aluminum, and solder joints between iron and aluminum have a definite thermal fatigue cycle life. As a result, iron-based metal connectors cannot be reliably soldered or laser welded to aluminum electronics packages for sustained periods of operation.
Interface between an aluminum electronics package and a standard iron-based metal connector may be accomplished through the use of a transition bushing fabricated from a dissimilar metal sheet consisting of an iron-based metal and an aluminum alloy. FIGS. 1a and 1b depict a standard iron-based metal connector 10 and a transition bushing 12, with the former including an integral, patterned arrangement of a plurality of pins, generally formed of iron-based metal, and the latter including an iron portion 14 and an aluminum portion 16. Transition bushing 12 surrounds the perimeter of standard connector 10, with iron portion 14 of transition bushing 12 affixed to a flange 18 of iron-based metal connector 10. After transition bushing 12-connector 10 attachment is accomplished, the combination is installed into an aluminum electronics package (not shown), with aluminum portion 16 of transition bushing 12 affixed to the aluminum electronics package to form an electronics assembly. In this manner, transition bushing 12 serves to provide a similar metal interface for both iron-based metal connector 10 and the aluminum electronics package.
Using transition bushings or like members for installing hermetic feedthrus in an electronics package has a number of drawbacks. Transition bushings require the electronics package-connector interface(s) of an electronics assembly to be large, thereby impacting the space necessary for the connector to be housed within the transition bushing and the bushing, in turn, to be housed within the electronics package. For many applications, this size requirement is unacceptable, because the specified height of an electronics assembly is less than the corresponding dimension of the transition bushing required to house the connector.
Also, transition bushings are designed for use with standard iron-based metal hermetic connectors. Such connectors are relatively heavy, and more disproportionately so when used in combination with a light weight metal electronics package, e.g., an aluminum electronics package. The use of transition bushings adds to the number of electronics assembly components, thereby requiring additional assembly procedures. Moreover, deployment of transition bushings increases the linear length of the hermetic seal and, consequently, decreases the electronics assembly yield. Such problems contribute to the actual and effective cost of the electronics assembly.
Moreover, many standard iron-based metal connectors and/or transition bushings employed therewith are formed, at least in part, using magnetic iron-based metals. Such fabrication materials produce connectors having magnetic properties, which are undesirable in some connector applications. Also, the use of iron connecting pins limits the amount of current that a connector is capable of handling.
FIGS. 1c and 1d depict prior art radio frequency (RF) connectors, with FIG. 1c constituting a "spark plug" type and FIG. 1d constituting a "field replaceable" type. FIG. 1c shows a RF connector 10' with a hollow, exteriorly threaded stainless steel shell 12' having a KOVAR.RTM. glass-to-metal feedthru 14' affixed thereto by brazing at elevated temperature. Shell 12' also houses a teflon insert 16' having a pin socket 18' disposed therein at each longitudinal end thereof. A connector pin 20', generally formed of iron-based metal, inserts into pin socket 18'. A teflon member 22' surrounds connector pin 20' in longitudinal juxtaposition to shell 12', and a double knife edge seal ring 24' is disposed in circumferential juxtaposition to shell 12'. Ring 24' is formed of an iron-based metal, such as KOVAR.RTM. or stainless steel, and is optionally coated with silver.
To affix RF connector 10' to an interiorly threaded electronics package 26', torque (approximately 25 in-lbs) is applied to connector 10'. This force causes seal ring 24' to slightly cut into both connector 10' and electronics package 26', thereby creating a seal. To insure that connector 10' does not back out of electronics package 26' during transport or use, an edge 28' of a connector 10'-electronics package 26' assembly is soldered about the circumference of connector 10'. For this purpose, gold plating is optionally used to improve the wetting properties of the solder.
This seal is not a reliable hermetic seal, however. The two dissimilar metals, i.e., the externally threaded iron-based metal and the internally threaded aluminum metal, are in intimate contact at ambient temperature. Since aluminum has a higher expansion rate than KOVAR.RTM. or stainless steel, temperatures lower than ambient cause package 26' to squeeze connector 10', while temperatures higher than ambient produce a separation between those components. Such phenomena result in fatigue of the solder joint during thermal cycle and cause less than intimate contact between seal ring 24' and electronics package 26' and between seal ring 24' and connector 10'. The external solder application at 28' to prevent connector 10' backout provides a mechanical lock between the components rather than a hermetic seal. The connector is not field replaceable because removal thereof compromises the hermeticity of the package and breaks the rigid connection to the end of the pin located inside the package. That is, connector 10' cannot be replaced in the field without a high risk of electronics package 26' circuitry compromise.
In addition, the electrical performance of RF connector 10' suffers as a result of temporal separation between the communication of the signal and the ground to electronics package 26'. The signal follows an essentially straight line path through connector 10' into electronics package 26'. In contrast, the ground path runs along the outer surface of teflon insert 16', the outer surface of the glass portion of feedthru 14', the outer surface of teflon member 22', through seal ring 24' into electronics package 26' and about the periphery of the interior of package 26' to the ground location therewithin. The ground lag caused by the disparity in signal/ground path lengths impacts signal gain and loss characteristics, thereby affecting the signal-to-noise ratio. This problem is exacerbated as higher frequency signals are employed.
A "field replaceable" RF connector 30', as shown in FIG. 1d, includes an exteriorly threaded, replaceable portion 32' formed of stainless steel. A KOVAR.RTM. glass-to-metal feedthru 34' is soldered into a cavity 36' in an aluminum electronics package 38' at one or more solder locations 40'. Replaceable portion 32' is torqued into an interiorly threaded aluminum portion 42'.
KOVAR.RTM. and aluminum exhibit an approximately 4:1 thermal expansion mismatch. As a result, seals using field replaceable connectors 30' are hermetic at ambient temperature only. The KOVAR.RTM.-aluminum solder seal fails during thermal cycle. Moreover, connector 30' does not meet military field replaceability standards (i.e., an iron-based metal part may be threaded into aluminum only once, because that operation impacts subsequent torque applications by displacing the aluminum in the threaded area).
As discussed with respect to prior art micro-D connector designs, the use of a magnetic material, such as KOVAR.RTM. or the like, in fabricating connectors imparts magnetic properties thereto. Such properties are not desirable in all connector applications.